A switched-mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of a power switch of the power converter. The duty cycle “D” is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent). Additionally, as the voltage or the current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value.
Power converters designed to operate at low power levels typically employ a flyback power train topology to achieve low manufacturing cost. A power converter with a low power rating designed to convert ac mains voltage to a regulated dc output voltage to power an electronic load such as a printer, modem, or personal computer is generally referred to as a “power adapter” or an “ac adapter.”
Power conversion efficiency for power adapters has become a significant marketing criterion, particularly since the publication of recent U.S. Energy Star specifications that require a power conversion efficiency of power adapters for personal computers to be at least 50 percent at very low levels of output power. The “One Watt Initiative” of the International Energy Agency is another energy saving initiative to reduce appliance standby power to one watt or less. These efficiency requirements at very low output power levels were established in view of the typical load presented by a printer in an idle or sleep mode, which is an operational state for a large fraction of the time for such devices in a home or office environment. A challenge for a power adapter designer is to provide a high level of power conversion efficiency (i.e., a low level of power adapter dissipation) over a wide range of output power.
Numerous strategies have been developed to reduce manufacturing costs and increase power conversion efficiency of power adapters over a wide range of output power levels, including the incorporation of a burst operating mode at very low output power levels. Other strategies include employing an energy-recovery snubber circuit or a custom integrated controller, and a carefully tailored specification. Each of these approaches, however, provides a cost or efficiency limitation that often fails to distinguish a particular vendor in the marketplace. Thus, despite continued size and cost reductions of components associated with power conversion, no satisfactory strategy has emerged to reduce power converter dissipation at low load currents.
Accordingly, what is needed in the art is a circuit and related method for a power converter that enables a further reduction in manufacturing cost while reducing power converter power dissipation, particularly at low load currents, that does not compromise end-product performance, and that can be advantageously adapted to high-volume manufacturing techniques for power adapters and other power supplies employing the same.